1. Field of the Invention
The present invention generally relates to packaging materials for products which are sensitive to static electric fields and discharge, and more particularly to a flexible, transparent film which shields against electrostatic fields, and further resists triboelectric generation of static charges and provides for the rapid dissipation of such charges.
2. Description of the Prior Art
Electrostatic discharge, as well as the mere presence of a static electric field, can be extremely detrimental to sensitive electronic parts. This is particularly true of modern semiconductors and integrated circuits which may be degraded or destroyed by the buildup of static electricity in the workplace. Especially sensitive components can be severely affected by an electrical potential as small as 50 volts, yet the simple act of walking has been shown to triboelectrically generate a potential of 30,000 volts or more.
One of the most common methods of protecting electronic components is to store, ship and handle them using pouches, bags, tote boxes or other packaging constructed of materials which provide shielding against electrostatic fields and other electromagnetic influences. These materials may additionally provide for the rapid dissipation of electrostatic charges which are applied to the package, and/or include a coating which prevents or minimizes triboelectric charging along the surface of the package. The present invention is directed to a film which may be fashioned into packaging providing such protection.
The most basic function provided by these types of films is protection from static electric fields. This is typically achieved by forming a conductive layer on the film or dispersing conductive materials therein, which effectively creates a Faraday cage. An example of a rather complicated process for creating a shielding film is disclosed in U.S. Pat. No. 4,645,566 issued to Kato et al. A thermoplastic, synthetic pulp is mixed with thermoplastic, composite fibers and conductive fibers, resulting in a paper-type stock. This stock may be formed into a web, and when it is heated and dried at appropriate temperatures, the pulp melts, leaving a film which has a network of conductive fibers dispersed therein. Other processes using a pulp of polymeric and conductive components are discussed in Japanese Patent Application Kokai Nos. 60-210667, 60-257234 and 62-42499.
The use of a web carrier in creating a conductive intermediate layer is common. In U.S. Pat. Nos. 4,939,027 and 4,983,452,both issued to Daimon et al., a moldable static-shielding sheet is formed by first obtaining a web of nonwoven fabric comprising conductive fibers and thermoplastic fibers. The web may alternatively be formed by irregularly twining a conductive fiber with a hot-melt adhesive fiber. The web is placed between two thermoplastic films and heated. This bonds the films to the web, and further causes the thermoplastic fibers in the web to melt and combine with the films, leaving an imbedded layer of conductive fibers. Other techniques utilizing a web or fabric are shown in Japanese Patent Application Kokai Nos. 60-34099, 60,143699, 62-47200, 62-122300, 62-275727, and 62-276297.
In addition to providing electrostatic shielding, these films may be treated to provide for the rapid dissipation of static charges, and to further minimize the triboelectric generation of such charges. In accordance with the American National Standards Institute EIA standard 541 ("Packaging Material Standards for ESD Sensitive Items," published June 1988) the first of these qualities is referred to as static-dissipative, and the latter is referred to as antistatic. For example, U.S. Pat. No. 4,623,594 issued to A. Keough teaches the use of an electron-beam curable resin composition which may be applied to a substrate, and which causes the antistatic properties to migrate through the substrate upon curing.
One product which combines the qualities of electrostatic shielding, static-dissipation and triboelectric charge resistance is described in U.S. Pat. Nos. 4,154,344 and 4,156,751, both issued to Yenni et al. Those patents describe a sheeting material which is formed by first applying a conductive material, such as nickel, to one surface of a biaxially oriented polymer substrate, namely, polyester. The exposed nickel surface is then coated by a solvent process with a protective layer. A film of heat-sealable material (polyethylene) is extruded onto the opposite surface of the polyester film. The extruded polyethylene may include antistatic material (such as quaternary ammonium compounds), or be further coated with such antistatic agents. Other conductive materials, such as carbon-loaded plastics, may be substituted for the metallic coating.
A similar product and process is described in U.S. Pat. No. 4,756,414 issued to C. Mott, except that the conductive layer is provided between the heat-sealable plastic and the second plastic layer. The conductive layer is applied by either vacuum deposition or sputtering techniques. The two plastic layers may be joined using a thermosetting adhesive. See also Japanese Patent Kokoku No. 38398/89. In U.S. Pat. No. 4,875,581 issued to Ray et al., a dielectric material is interposed between the two plastic layers and, in addition to a conductive layer, the outer elastomeric layer of the film includes additives which make it static-dissipative. Even more complicated processes and resulting structures are known, such as the film discussed in U.S. Pat. No. 4,906,494 issued to Babinec et al., which utilizes a two-ply layer having a polyolefin and a copolymer of ethylene-acrylic acid and ethylene-vinyl acetate. In Babinec, the layers are joined using an adhesive or hot roll lamination.
Another laminated film is disclosed in U.S. Pat. No. 3,888,711 issued to W. Breitner. Although that film is primarily intended for use as a heated window or alarm glass, Breitner notes that it may also be used for shielding against electromagnetic interference. The lamination process used requires a knit of thermoplastic and conductive filaments which is applied to a sheet of thermoplastic material. The thermoplastic filaments are preferably constructed of the same materials as the thermoplastic sheet; the thermoplastic filaments should at least be fusible with the sheet. When the sheet is fed through hot rolls, the thermoplastic filaments fuse with the sheet, leaving a network of conductive filaments forming a plurality of interconnected conductive paths. A cover sheet may be laid on top of the network before hot rolling. Another lamination technique (using conductive strands rather than a web) is depicted in Japanese Patent Application Kokai No. 60-81900.
Each of the foregoing films has its disadvantages. First and foremost of these is the poor transmission of visible light ("transmissivity") through these films and packaging formed therefrom, whether they rely on metal coatings, carbon-loading or filament networks. Previously, increasing transmissivity meant sacrificing static-shielding ability. The best transmissivity claimed by any of the foregoing films is 60%, although this has proven very difficult to achieve at production levels. Furthermore, while this level of transmissivity allows a visual inspection of the general condition of items contained in the packaging, it does not provide adequate legibility, i.e., the ability to read printed matter on the item in the packaging, particularly if the printing is not flush against the interior surface of the film.
With respect to films formed using webs, fabrics or pulp, those processes are generally more complicated and, hence, the resulting films are more expensive. The films typically have even less transmissivity than metal coatings, but such coating processes also add significant expense. Pulp-type films are conductive through their thickness, and so require additional process steps to insulate their surfaces. As noted by Kato et al., films resulting from these processes further have inferior physical and chemical bonding resulting in low tensile, tear and surface strength.
A thin, conductive filamentary network would overcome many of these problems but it has been difficult in the past to apply such a network without the use of webs or pulp. Breitner notes that applying metal strips is labor intensive, but his process still requires a special prefabricated knit (or special machinery to directly apply filaments to the film). Breitner further notes the difficulties in using very thin conductive filaments, which would be preferable since this would enhance transmissivity through the network. Another problem with static-shielding films using filaments is that the network must be relatively dense, which inhibits transmissivity. For example, Daimon et al. ('027) teaches that the network must have minimum density of 15 g/m.sup.2. Finally, sandwiching a conductive network between two polymeric layers, often results in wrinkles and/or voids at the interface where the filaments lay. This affects transmissivity and legibility, and may also increase the likelihood of peeling.
Several of the above limitations have been overcome by the film described in U.S. patent application Ser. No. 07/270,532, now U.S. Pat. No. 5,028,490 (D. Koskenmaki), assigned to Minnesota Mining and Manufacturing Co. (3M), assignee of the present invention. In that patent, which probably represents the closest prior art, molten strands of conductive material are squirted directly onto a polymeric substrate, which may then be further laminated. Nevertheless, the equipment necessary to produce molten filaments is relatively expensive, and presents clear hazards. Also, since the exemplary laminated films of Koskenmaki create an interface at the filamentary network, they are more likely to have wrinkles and/or voids, and more prone to peeling at the interface. Therefore, it would be desirable and advantageous to provide a static-shielding film having a very sparse network of thin conductive filaments to achieve improved transmissivity and legibility, the filaments further being suspended in a polymeric layer in such a manner that there is no discernable interface surrounding the filaments even though they form an essentially two-dimensional conductive network. The filaments should further not be applied in a molten state.